That is an issue that has been around for a while... when you download a media player it wants to 'own' all of the file types it can play... even if it takes the assosiation away from another program you may already have. It isn't just Quicktime... most, if not all, do it. Several offer an option to select what type of media that you want it to deal with when you are installing it so you might want to keep an eye on it the next time you install.

In any event as stated you should be able to change the association back with no major worries.